1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for profile selection during fast retrain of digital subscriber line modems operating in communication systems using the discrete multi-tone standard.
2. Discussion of the Related Art
In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, public switched telephone network (PSTN) information, as well as, information from ancillary equipment in analog form (i.e. computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have employed discrete multi-tone, hereinafter DMT, systems to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines. The provision of asynchronous digital subscriber lines (ADSL) using DMT systems to customer premises has proliferated over recent years. Since ADSL signals are transmitted in a higher frequency band than that of the POTS frequency band, transmitting signals from both the POTS and ADSL frequency bands over the same twisted-pair telephone line (even at the same time), generally is not a problem. Specifically, the POTS frequency band is generally defined from 0 Hz to 4 kHz, while ADSL frequency bands are generally defined by a lower cutoff frequency of approximately 26 kHz, and an upper cutoff frequency of approximately 1 MHz.
In the past, a combination of circuits termed hybrids, and POTS splitters have served to buffer ADSL equipment from distortions and interference introduced in the ADSL frequency bands from the lower frequency POTS equipment. In a DMT-G.Lite standard configuration, the POTS splitter is no longer present. As a result, POTS equipment operates on the same twisted-pair phone line that is being used to deliver ADSL services. POTS equipment operating in this configuration is subject to interference from low frequency harmonics generated within the ADSL equipment.
Conversely, and of greater significance, the presence of abrupt changes in line conditions due to ringing, customer premises noise, POTS handset pick-up, and on/off-hook transitions from ancillary equipment, can disrupt ADSL transmissions. Splitterless operation of an ADSL communication system often incurs a significant and abrupt insertion-loss change upon the off-hook terminating impedance change of the POTS device. As a result, there is a need for a fast recovery mechanism to cope with POTS transients in the ADSL splitterless DMT communication system. DMT systems, by nature of their distribution across multiple frequency bands, are capable of retuning devices to optimize data transfer for changing line conditions. DMT devices selectively transfer bits from the data stream in those discrete frequency bands that are uncorrupted from amplitude modulation radio interference and unaffected by phone system bridge taps, thereby tuning, or maximizing performance under changing line conditions.
Tuning of DMT system parameters is currently performed in two distinct ways:
initial training, hereinafter called xe2x80x9cfull retrain,xe2x80x9d and bit loading/swapping, an online optimization procedure. Another often suggested means to retune a system is a fast retrain of the connection. A full retrain of the system connection results in a temporary loss of service and is undesirable under most conditions. Of the methods used to tune DMT parameters, fast retrain is best suited to overcome transient effects, while bit loading/swapping is more adapted to slowly varying changes. The fast retrain method is more robust than bit loading/swapping and provides for a more optimized system since it can actively readapt other system components such as equalizers and echo-cancelers to the system noise environment.
The fast retrain algorithm is triggered when either the central office or the remote transmission unit sense the need to transition from the current parameter profile to a more appropriate previously stored parameter profile. The most typical situation that triggers a fast retrain is when a POTS device goes on/off hook. These transitions create impedance transients that adversely affect the ADSL frequency spectra.
In telephony, an off-hook condition exists when an operational telephone instrument is in use, i.e., during dialing or communicating. xe2x80x9cOff-hookxe2x80x9d originated from the description of the above condition when the handset was removed from the switchhook which connected the handset to the line when the switch was not depressed from the weight of the handset. Today, xe2x80x9coff-hookxe2x80x9d pertains to the operating state of a communications link wherein data transmission is enabled for voice, data communications, or network signaling.
On/Off-hook transitions have various effects on transmitted signals. Some of these effects are time dependent, others are time independent. In a first approximation, the following phenomena can be distinguished: attenuation of the received signal, phase of the received signal, time dependency of the impairments, increase in the noise level, and a DC surge.
Fast retrain procedures are usually based upon stored profiles. It is assumed that previous full initialization procedures have been successfully completed upon earlier off-hook and on-hook transitions of POTS equipment at the customer premises. If the full initialization profile under such conditions has been stored in memory, a fast retrain can take advantage of that previous work by identifying current line conditions, recognizing if a suitable profile exists in memory, and simply recalling and applying the stored profile parameters.
Accordingly, it is desired to provide a system and method that efficiently, accurately, and quickly detects a suitable stored fast retrain profile to permit the resumption of ADSL communications in the presence of changing line conditions in a dual POTS/ADSL communications system.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and advantages of the present invention, the present invention is directed to a system and a method for the detection and application of a suitable stored fast retrain profile for a broadband modem using the DMT communication standard. When a suitable profile is not available or there is insufficient evidence to support the implementation of one of the stored profiles, a full system initialization procedure is triggered, skipping the fast retrain process.
The system and method of the present invention models on/off hook transitions and establishes an array of possible profile states. Given the current profile, the probability of transitioning to any of the stored fast retrain profiles is calculated and the results transformed into a probability distance between available profiles. Having established a distance relationship modeling the probability of selecting an appropriate profile, the decision process can be implemented through any of a number of different distance criteria. Once the system has determined a suitable fast retrain profile exists and has selected an appropriate profile, the stored profile parameters are used to configure the modem.